Reaper by John G. Hartness

What’s the first rule of fight club?
Don’t invite the friggin’ Reaper to fight club!

Cryptids are turning up dead all over Charlotte, and this time it’s not Harker’s fault. He’s tasked with finding out who is killing shifters and leaving their bodies in conspicuous places, and the best way to do that is to go undercover where the killings are taking place, find out who’s in charge, and persuade them to stop.
Persuade them in the very Quincy Harker way, of course.
So Harker hides his identity and gets himself conscripted as one of the combatants in Charlotte’s very own cryptid cage fighting arena, battling it out with vampires, werewolves, and faeries, oh my! Along the way he makes a friend (yeah, pretty much only one—have you met Harker?), learns to embrace the nickname he’s always hated, and throws down in some truly epic battles. Then he squares off against an uber-powerful vampire in the Main Event, and business really picks up.

When Harker finally figures out who’s running this ring of lethal combat sports, the answer is one he never suspects and leads to a final confrontation he never wanted. But sometimes, when the chips are down, you really should in fact fear the Reaper…